By deleting data manually or by doing Factory reset your personal data would not be deleted from phone permanently ,because the deleted data from a smartphone can be recovered. So even if you try to remove it, pass on the phone device with bits of information that may end up in the wrong hands,i recommend the Erasephone to you , which can help you wipe out android data thorough,

As i can see you want to completely delete android messages ,why not using androdi data erase tool, it is very likely for you not to remember all the info you have stored on it. So, there are high chances that you might forget to delete something, and pass on the mobile device with bits of information that may end up in the wrong hands, if you use the tool to erase android data , the phone data can be disappeared from phone permanently ,